I tried a similar experiment with Infinity Primus 150's (which I currently employ as speakers in the living room being fed by a Yamaha HTR-N5060).

I couldnt consistently tell the difference between vertical or horizontal placement. The only change I could consistently pick out was when the speaker was oriented upside down vs right side up in vertical placement.

Try this with a capable partner (and a few beer). Dont be shy and get the blindfold out. With the right person, Beer, blindfolds and music makes for a fun hour....

How is it double blind if one person is clearly able to tell a difference when he sets the speaker?

I'm all for beer-based testing sessions, of course. (In fact, right now I've got some organ music playing at earth-shaking volumes after having a few.) But there's nothing especially double blind about it that I know of.

On another note, I'm going to guess that certain rooms can tolerate this a lot more than others. Specifically, those that are kind of poor for sound in general. In them, horizontal vs vertical probably matters a lot less, as a (total guess here) 10% reduction in quality in a room that only lets you achieve 50% quality anyway is much less noticeably different than a perfectly engineered room.

How is it double blind if one person is clearly able to tell a difference when he sets the speaker?

Double blind studies are used to rule out actual differences from prejudice.

If you don't know the how's and why's, here's the short of it.

The "double" means both the tester and testee, are unaware of which sample is being tested. Or if the tester does know there's no way for that information to be communicated to the testee (that means no forms of communication between the two, not even non-verbal forms like body language).

The two samples are presented. The testee is then tasked with identifying each one uniquely after each switch. If he is only correct around 50% of the time, then it is just chance. But if one sample can be identified a majority of the time, then there is a difference.

Two very difference speakers will obviously be able to be identified uniquely.